Recently, foreign media newatlas published a detailed discussion of Rosetta’s bold plan to land on the comet for the first time. Just as archaeologists used their tools to target matter on Earth to find clues about the Earth’s distant past, studying ancient remains floating in space could also help reveal key secrets of the early solar system.
Launch date: March 2004
Current position: Frozen on 67P/Churyumov-Gerasimenko surface
But what happens when the object is a rubber duck-like comet that rolls in space at 135,000 km/h, and the archaeologist is a refrigerator-sized robot that needs to land somehow? This is a dilemma for ESA scientists as they embark on an unprecedented mission to study comet 67P/Churyumov-Gerasimenko.
Although there have been precedents for comet flying before, no one had entered the comet’s orbit at the beginning of the ESA Rosetta mission, let alone try to land on it. For the space agency, the challenge is due to the fact that the comet is essentially a time capsule, with large chunks of ice, rock and dust frozen on it that locked up the key material of the solar system’s formation 4.5 billion years ago.
So, in addition to studying the comet’s surface, an in-depth study of the comet’s subsurface material could also help shed light on the answers to how Earth and other planets in the solar system formed. In addition, as it approaches the sun and begins to release its highly safe material, it also forms its iconic tail and the atmosphere known as the comet.
But how do you catch it? Scientists have meticulously planned the Rosetta probe’s journey from Earth to 67P/Churyumov-Gerasimenko, but it’s clear that’s not enough, and there’s still a lot of work to be done during the flight.
Rosetta is understood to have lifted off in March 2004 with an Ariane 5 G-plus rocket, but there was not enough thrust to get it into orbit at 67P/Churyumov-Gerasimenko. Instead, Rosetta carried out a Mars flyby, three Earth overflights and a slingshot effect around Jupiter to the comet, a series that took about 10 years. During this time, mission control has been adjusting Rosetta’s orbit, and it is understood that the command from Earth takes 50 minutes to reach the probe. When Rosetta finally reached 67P/Churyumov-Gerasimenko, it finally ended its 6.4 billion-kilometer journey and 31 months of hibernation.
Mission scientists knew little about the comet’s surface until Rosetta made history and entered orbit 67P/Churyumov-Gerasimenko in August 2014. But that quickly changed, and as the probe aimed at its 4km-wide target, a more uneven world than had been expected began to slowly emerge.
Early observations of Rosetta in the triangular orbit showed that the comet lost water at a rate of 300 milliliters per second and had an average surface temperature of -70 degrees Celsius, suggesting that it was a particularly dark, dirty comet that absorbed sunlight rather than reflective.
Starting from this place, Rosetta’s imaging instruments began to determine the location of the 100-kilogram Philae lander. The refrigerator-sized robotic lab is filled with scientific instruments including cameras, imaging systems, spectrometers, gas analyzers and measuring acoustics. Importantly, it also has a drilling hole system that can drill 9 inches below the comet’s surface to collect samples to analyze the comet’s chemical composition.
But dropping a lander to the surface of a comet is a completely different challenge than landing on a planet or satellite. As Rosetta orbited 67P/Churyumov-Gerasimenko and mapped its surface, the mission control center weighed the options in the hope of finding the greatest possible scientific return without endangering mission safety.
According to scientists, the landing site requires a square kilometer of clean space, there are no slopes, large rocks and other risk factors, and a lander that can be exposed to sunlight to get energy. In addition, it needs a clear location for wireless communication with Rosetta. Most importantly, the team needs to avoid landing when the comet approaches the sun, where the gas released is probably the most dangerous obstacle.
Eventually, the scientists chose the J site as Philae’s soft landing site. On November 12, 2014, the team released Philae from its mother ship and sent it to the surface of the comet.
Without power, the lander drifted in the direction of 67P/Churyumov-Gerasimenko by relying on the comet’s extremely weak gravitational field. The process lasted more than seven hours, and in the process, the lander took a farewell image and measured the magnetic field along the way.
When landing on the comet’s surface, a set of drill bits drilled holes in the comet and fixed Philae in place with a harpoon — at least as the mission control center thought. Soon after, however, the control center learned from telemetry that the anchoring device had actually failed, causing the lander to bounce back into space, and once and twice. Eventually, it was trapped in a hole in Point J, about 1 km from where it had been planned.
At this point, Philae lies sideways in the shadows, its three legs only two touching the comet’s surface, and some of its solar panels are covered with comet dust so they can’t get the solar energy needed to keep the batteries charging and the instrument sit.
Philae can run for about 60 hours on batteries that are already fully charged. To do this, the team instructed the lander to activate all instruments and send back as much data as possible via Rosetta. In this relatively small window of discovery, Philae actually completed 80 percent of the scientific research it originally set and sent valuable data back to Earth. In a way, it was also the first drilling on a comet.
Although these instructions were completed, the results of subsequent analyses were regrettable because the drilling instrument did not actually reach the ground, so it had no scientific significance.
When the battery runs out, Philae goes into sleep mode. About six months later, the ESA instructed Rosetta to send a wake-up signal to the sleeping lander over an eight-day period, but received no response.
A few months later, in June 2015, Philae miraculously showed signs of life, contacting it intermittently eight times over a period of nearly four weeks. In addition to these sporadic signals, the lander has been isolated, and in July 2016, ESA decided to say goodbye to it forever.
While part of Philae’s mission may always live as THE ESA originally hoped, landing on a comet — albeit a bit clumsy — is still a milestone in space exploration. Rosetta, which is still in the 67P/Churyumov-Gerasimenko orbit, is continuing to collect scientific data on the comet.
Through Rosetta’s observations, it is now known that there is an active sinkhole on comet 67P/Churyumov-Gerasimenko and that the comet smells like a stinky egg — a smell of ammonia, hydrogen and other chemicals from the comet’s surface. In addition, the comet also had a temporary small satellite, the comet’s color is blacker than charcoal. In addition, its atmosphere contains key amino acids and molecules that are considered the cornerstones of life, and it also has a large amount of water ice on its surface.
Rosetta gently crashed into 67P/Churyumov-Gerasimenko in September 2016, and it continued to collect data until the last minute, bringing a dramatic end to its 12-year historic journey.